1. Field of the Invention
The present invention relates to semiconductor devices which take advantage of ballistic transport of energetic carriers.
2. Description of the Prior Art
The switching speed of the transistors that lie at the heart of most electronic circuits is limited by the electron or hole velocity. Usually, the motion of carriers (electrons and holes) in semiconductor devices is constantly interrupted by scattering. Each carrier is scattered frequently and in every direction so that the net flow of electrons in the direction of electric field is much slower than the actual speed of the electrons. If it were possible to enable electrons in semiconductors to move without scattering--i.e. to move ballistically--then the speed of transistors could be increased dramatically.
Ballistic motion of electrons in semiconductors was predicted in M. Shur and L. Eastman, IEEE Trans. Electron. Device ED-26, 1677 (1979). This model states that under certain conditions, the ballistic transport of electrons can be realized in short semiconductor devices (i.e. ballistic devices). Considerable effort has been directed toward the development of such devices, and numerous theoretical and experimental studies have pointed to the existence of the ballistic transport mechanism.
Only recently has a direct confirmation of ballistic motion been obtained by two groups based on the studies of the electron transport in a short base region of a high electron transistor (HET) and a planar doped barrier transistor (PDBT). M. Heiblum, M. I. Nathan, D. C. Thomas and C. M. Knoedler, Phys. Rev. Lett. 55, No. 2,2200 (1985); and A. F. J. Levi, J. R. Hayes, P. M. Platzman and W. Wiegmann, Phys. Rev. Lett. 55, No. 19, 2071 (1985).
The device structures which make use of ballistic transport typically include a ballistic electron injector for launching high energy (hot) electrons; a highly doped base region which is thinner or approximately equal to the mean free path of hot electrons; and a collector with an associated barrier which prevents cold electrons from passing while allowing hot electrons to pass from the base to the collector. The ballistic electron injector can be formed, for example, by a heterojunction, a planar doped barrier, or a tunnel barrier. In each case, a barrier is formed between an emitter and the base so that the electrons reaching the base region have a high potential energy. Since the base is a region of relatively low potential energy, the kinetic energy of the injected electrons increases as their potential energy decreases.
Despite a high level of interest in high speed semiconductor devices, and considerable theoretical and experimental efforts directed to the use of ballistic transport of electrons in semiconductors, development of practical ballistic devices is still at a very early state. Material problems, technology difficulties and lack of good means for effective launching and collecting of ballistic electrons have all limited the performance of proposed and experimentally built devices.